According to Ian Rapoport of NFL Network, WR Mike Williams will receive an MRI on his knee to test it with the fear being that he suffered a significant, potentially season-ending, injury. (Ian Rapoport on Twitter)
The hope is that his injury will be lesser than the initial fears, but we will not know until we get back the results. If Williams misses any significant time, rookie WR Quentin Johnston will likely see a significant increase in role. He has played behind WR Joshua Palmer, who will likely pair with Keenan Allen in two-WR sets, but could see increased usage in the passing game.